Remove WebView fallback package handling.

There is no longer a special "fallback package" state for WebView that
means it should not be able to have its enabled/disabled state
controlled by the user. Remove the code used to implement this in
Settings.

Bug: 129470358
Test: verify that WebView's enable/disable button is not greyed out
Change-Id: Ifc0921511a71282a77f239e5ff5955e60fab6e2c